1. Field of the Invention
This invention relates to four wheel drive working vehicles such as lawn mowers, typically a front mower.
2. Description of the Related Art
With a four wheel drive working vehicle having rear wheels acting as dirigible wheels and being the part-time type without a center differential, for example, the rear wheel differential has a larger turning radius than the front wheel differential in time of making a turn in four wheel drive mode. In such a situation, the differential motion between the front wheels and the rear wheels cannot be absorbed, resulting in a tight corner braking phenomenon to hamper the small turn. However, it is very troublesome to break drive transmission to the front wheel differential or rear wheel differential each time a turn is made.
Conventionally, as shown in FIG. 6, a transmission line from a transmission case 7 to a front wheel differential 8 and a rear wheel differential 8a includes a dual one-way clutch 14 disposed in a position of the center differential.
In time of forward running, the rear wheel differential 8a has a turning radius Rr which is larger than a turning radius Rf of front wheel differential 8. Consequently, an output shaft 100 extending to the rear wheel differential 8a has a higher forward rotating rate than an input shaft 101 interlocked to the front wheel differential 8. At this time, the dual one-way clutch 14 breaks the transmission of forward drive from the input shaft 101 to the output shaft 100. In time of backward running also, the turning radius Rr of rear wheel differential 8a is larger than the turning radius Rf of front wheel differential 8. Consequently, the dual one-way clutch 14 breaks the transmission of backward drive from the input shaft 101 to the output shaft 100 when the backward rotating rate of output shaft 100 is higher than the backward rotating rate of input shaft 101.
In time of straight running, drive is transmitted from engine 4A to front wheels 1 and rear wheels 2 to produce a four wheel drive mode. When making a turn, the drive transmission to the rear wheels 2 acting as dirigible wheels S is broken to produce a two wheel drive mode.
With the above conventional construction, automatic switching is made to the two wheel drive mode when making a turn, and to the four wheel drive mode when running straight after making the turn. This provides excellent operability for making small turns. However, there is a drawback in that the driving force tends to be insufficient when making a turn in the two wheel drive mode.
The object of this invention is to provide a construction for making a small turn with a sufficient driving force.
The above object is fulfilled, according to this invention, by a four wheel drive working vehicle comprising a vehicle body, a pair of dirigible wheels supported by the vehicle body, a further pair of wheels supported by the vehicle body, an engine supported by the vehicle body, and transmission means for transmitting drive from the engine to the pair of dirigible wheels. The transmission means has a clutch mechanism operable, when the vehicle makes a turn, to break drive transmission to one of the dirigible wheels lying outwardly with respect to a turn center, and to maintain the drive transmission to the other dirigible wheel lying inwardly with respect to the turn center.
An inconvenience encountered when the vehicle makes a turn in a four wheel drive mode is that the outer dirigible wheel having the largest turning radius among the four wheels rotates too slowly. This outer dirigible wheel brakes the vehicle making the turn.
With this in view, the clutch mechanism is provided to break the drive transmission only to the outer dirigible wheel when the vehicle makes a turn. Thus, in time of turning, the outer dirigible wheel is placed in a state of free rotation. The vehicle can make a turn in a three wheel drive mode without being braked by the outer dirigible wheel.
Consequently, a small turn can be made in a strong and reliable way with a sufficient driving force based on three wheel drive.
Other features and advantages of this invention will be apparent from the following description of the preferred embodiment to be taken with reference to the drawings.